(CNN) -- A lawyer in Portland, Oregon, has been taken into custody by the FBI in connection with the March 11 terror attacks in Madrid, law enforcement sources told CNN.

Attorney Brandon Mayfield's detention on Thursday was based on information from Spanish authorities that the lawyer -- who the sources said had converted to Islam -- may have been involved in the train bombings that killed 190 people.

According to the sources, Mayfield's fingerprints were found on a plastic bag with bomb-related material that had been connected to the Madrid attack.

The sources said Mayfield, 37, is being called a material witness, which means he can be held and not charged.

"I know he is innocent. Everyone knows he's innocent," Mayfield's wife, Mona, told a local television station KGW.

She said her husband is probably thinking, " 'This is just appalling, this is ridiculous, these kind of charges.' "

A spokeswoman for the FBI in Portland told CNN that agents served two search warrants Thursday in Washington County but would not comment further.

Mayfield had been under around-the-clock surveillance by law enforcement, the sources said.

Mayfield represented one of the chief defendants in the so-called "Portland Seven" terror case in a custody battle.

Six members of the alleged Portland terror cell pleaded guilty in October of plotting to fight with the Taliban against the United States in Afghanistan.